The present invention generally relates to implantable cardiac stimulation systems and other types of implantable medical devices. Particularly, this invention relates to a method of protecting implanted devices from the effects of external magnetic fields associated with external diagnostic/programmer systems that could result in damage to the implanted devices. More specifically, the present invention relates to an integrated circuit coupled to a high frequency carrier transformer in which the functionality of the integrated circuit prevents damage to the switches in the implanted devices by blocking driving levels with insufficient amplitude. Furthermore, with the inclusion of additional logic circuitry within the integrated circuit, the invention can control more than one protected output using only one transformer, thus contributing to a reduction in the device size.
Implantable devices are implanted in a human or animal for the purpose of performing a desired function. This function may be purely observational or experimental in nature, such as monitoring certain body functions; or it may be therapeutic or regulatory in nature, such as providing critical electrical stimulation pulses to certain body tissue, nerves or organs for the purpose of causing a desired response. Implantable medical devices such as pacemakers, perform both observational and regulatory functions, i.e., they monitor the heart to ensure it beats at appropriate intervals; and if not, they cause an electrical stimulation pulse to be delivered to the heart in an attempt to force the heart to beat at an appropriate rate. In some cases, a number of functions are required for the patient""s well being. With space at a premium, it is desirable that multiple functions be incorporated into a single device.
An implantable device, such as a pacemaker, must perform its functions at minimum inconvenience and risk to the person or animal within whom it is used. It must be long-lived and reliable. In most cases, the volume must be minimized. Typically, a noninvasive telemetry system must be provided to allow data and commands to be readily transmitted between the implantable device and an external programmer. The external programmer provides a convenient mechanism through which the operation of the implantable device can be controlled and monitored, and through which data sensed or detected by the implantable device can be transferred out of the implantable device to an external (non-implanted) location where it can be read, interpreted, or otherwise used in a constructive manner.
A permanent magnet can be placed over the implantable device to enable the transmission of specific commands to the implantable device. The implanted device senses the external magnetic field using a reed switch or a special magnetic sensor.
However, the strong magnetic field associated with the external magnet might adversely affect key components in the implantable device. In particular, the strong magnetic field of the external magnet might have unintended, deleterious effects on the high frequency carrier transformer and, ultimately, on the HV (high voltage) switching elements within the device.
In an implantable cardioverter/defibrillator (ICD), key elements in the generation and application of the high voltage electrical stimulation pulses are the main electrical switches that discharge electrical energy (ranging from less than 0.5 Joules to as much as 40 Joules) into the appropriate regions of the heart. These switches, typically power MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors). If an insufficient IGBT drive voltage is applied, the IGBT can be destroyed, thus rendering the implantable device non-functional.
Drive voltages for the gates of the main switches are derived from the rectified output of the secondary of a high frequency carrier transformer. Reference is made to U.S. Pat. No. 4,800,883 to Winstrom. The use of a 2 MHz carrier frequency allows the core of the transformer to be substantially reduced in size, as required by the constraints on the volume of the implantable device.
However, the reduction in the size of the core leaves the implantable device susceptible to the effects of external magnetic fields such as those represented by an external magnet. In particular, if the core is subjected to a magnetic field of sufficient magnitude and/or asymmetry, the volt-second product of the core may be exceeded. As a result, the core saturates and the output of the secondary decreases due to the reduced coupling factor between the primary and secondary windings. This reduced secondary voltage may be insufficient to effectively and safely drive the gates of the MOSFETs/IGBTs, and might result in damage to or destruction of these switching elements.
Ideally, the isolation transformer is not used while the external magnet is present and, as such, implantable devices are typically equipped with a magnetic field sensor such as a reed switch or Hall effect device that inhibits triggers to the main switching elements. However, it is possible that the magnetic sensor in the implantable device may not sense the field because of dead zones or field nulls near the sensor. For example, even a strong magnetic field perpendicular to the reed switch would not actuate the latter.
If the implantable device attempts to turn on the MOSFETs/IGBTs, and if the transformer supplying the drive to the gate of the switches is saturated, the voltage at the secondary of the transformer may be below the required value, and the main switches may be damaged or destroyed. An IGBT typically requires 15V between the emitter and the gate to be fully ON. While fully on, and at a delivery current of 16 A (e.g. 800V at a 50-Ohm body impedance), the IGBT voltage drop (collector to emitter) is approximately 2V. In this case, for the duration of the applied shock of approximately 5 ms, the IGBT needs to sustain 32 W (e.g. 2V*16A). Considering that the IGBT is insufficiently driven such that the voltage drop is 40V. The current in the circuit would then be (800-40)V/50 Ohm=15.2A. The new power dissipation will be 40V*15.2A=608W, which will destroy the IGBT.
There is therefore a still unsatisfied need for a system that protects the implantable device from an external magnetic field that might saturate a transformer core and lead to damage or destruction of MOSFET/IGBT main switches therein.
The protection system of the present invention addresses and satisfies this need. The protection system includes one or more control blocks that inhibit gate triggers for the switching elements (IGBTs) of an H-bridge whenever the drive voltage for these switching elements falls below a pre-determined level. Each control block provides a controlled path to charge the IGBT gate when triggering is required and, importantly, when the gate drive voltage is above a predetermined threshold.
According to a preferred embodiment, gate triggers for the MOSFETs/IGBTs are inhibited whenever the drive voltage for the MOSFETs/IGBTs falls below a pre-determined level. This reduced voltage may be due to insufficient secondary voltage resulting from a saturated transformer core or from insufficient primary voltage or even defective components. With the inclusion of readily added logic circuitry, the functionality of the invention can be extended to control more than one output that needs to be controlled at the same isolated voltage.
The foregoing and other features of the present invention are achieved by implementing a protection system that employs one or more control blocks in conjunction with a high frequency carrier transformer. The control block, in its most basic implementation, contains logic and control circuitry that inhibits low voltage gate drive pulses that could result in damage to or destruction of the MOSFETs/IGBTs in an implantable device. The control block prevents inadvertent or spurious triggering and protects the gate circuitry of the MOSFETs/IGBTs by providing a low impedance path between the gate and source of the MOSFETs/IGBTs when the devices are intended to be turned off. Additionally, it protects the MOSFETs/IGBTs from over-voltage effects by clamping the output of the gate trigger with a shunt regulator circuit.
The control block is readily produced in a typical 2 xe2x96xa1m N-well CMOS process. This technology also includes bipolar transistors, isolated vertical npn transistors and substrate vertical pnp transistors.
Briefly, the operation of the circuit may be summarized as follows: A pulse-code modulated 2 MHz square wave is applied to the primary of a high frequency isolation transformer. The transformer is equipped with a magnetic core to enhance the primary-to-secondary coupling. The output of the secondary is half-wave rectified and is used to pulse-charge a capacitor. During the initial application of the square wave to the transformer primary, the logic circuitry, powered by a voltage derived from rectified secondary output, goes through a transient reset operation to ensure that all logic values are in well-defined states.
At this point, the embedded threshold detector determines whether the available voltage is sufficient for MOSFET/IGBT to be turned ON. In the case where the voltage exceeds the threshold level, a level signal is stored when a brief carrier interruption occurs. After the short interruption, the transformer is again energized and the IGBT is turned ON, only if signal level signal exceeds the threshold. After the IGBT is turned ON, it remains ON as long as the 2 MHz carrier is applied. When this carrier is interrupted, the circuit using three transistors that are configured as a triple Darlington (also referred to as a quick discharge circuit), are triggered, quickly discharging the IGBT gate to emitter capacitance. In addition, the circuit provides a low impedance path between the gate and source of the MOSFET/IGBT. This low impedance effectively renders the MOSFETs/IGBTs untriggerable, and protects them from inadvertent triggers and/or high voltage transients.
The protection system also includes an over-voltage protection scheme that clamps the charge voltage by means of a shunt regulator circuit.
Yet another feature of the invention is its ability to readily accommodate additional output capabilities. For example, an output, not reliant on the interrupted carrier sequence described above, could be available anytime the carrier frequency is applied. An identified use for such an output would be a means of discharging the high voltage capacitors (known as a DUMP function) of a defibrillator included in the pacemaker. Thus, a single transformer is capable of controlling both the shock and dump functions of the implantable device. The flexibility of the invention still allows a magnetic field sensor, such as a reed switch or Hall-effect probe, to be incorporated in the implanted device.